Third generation (3G) mobile cellular systems, such as, for instance, universal mobile telecommunication systems (UMTS) standardized within the third generation partnership project (3GPP) have been based on wideband code division multiple access (WCDMA) radio access technology. Today, 3G systems are being deployed on a broad scale all around the world. After enhancing this technology by introducing high-speed downlink packet access (HSDPA) and an enhanced uplink, also referred to as high-speed uplink packet access (HSUPA), the next major step in evolution of the UMTS standard has brought the combination of orthogonal frequency division multiplexing (OFDM) for the downlink and single carrier frequency division multiplexing access (SC-FDMA) for the uplink. This system has been named long term evolution (LTE) since it has been intended to cope with future technology evolutions.
The LTE system represents efficient packet based radio access and radio access networks that provide full IP-based functionalities with low latency and low cost. The downlink supports data modulation schemes QPSK, 16QAM, and 64QAM and the uplink supports QPSK, 16QAM, and at least for some devices also 64QAM, for physical data channel transmissions. The term “downlink” denotes direction from the network to the terminal. The term “uplink” denotes direction from the terminal to the network.
LTE's network access is extremely flexible, using a number of defined channel bandwidths between 1.4 and 20 MHz, contrasted with UMTS terrestrial radio access (UTRA) fixed 5 MHz channels. Spectral efficiency is increased by up to four-fold compared with UTRA, and improvements in architecture and signaling reduce round-trip latency. Multiple Input/Multiple Output (MIMO) antenna technology should enable 10 times as many users per cell as 3GPP's original WCDMA radio access technology. To suit as many frequency band allocation arrangements as possible, both paired (frequency division duplex FDD) and unpaired (time division duplex TDD) band operation is supported. LTE can co-exist with earlier 3GPP radio technologies, even in adjacent channels, and calls can be handed over to and from all 3GPP's previous radio access technologies.
An LTE network architecture including network entities and interfaces between them is exemplified in FIG. 1. As can be seen in FIG. 1, the LTE architecture supports interconnection of different radio access networks (RAN) such as UTRAN or GERAN (GSM EDGE Radio Access Network), which are connected to the EPC via the Serving GPRS Support Node (SGSN). In a 3GPP mobile network, the mobile terminal 110 (called User Equipment, UE, or device) is attached to the access network via the Node B (NB) in the UTRAN and via the evolved Node B (eNB) in the E-UTRAN access. The NB and eNB 120 entities are known as base station in other mobile networks. There are two data packet gateways located in the EPS for supporting the UE mobility—Serving Gateway (SGW) 130 and Packet Data Network Gateway 160 (PDN-GW or shortly PGW). Assuming the E-UTRAN access, the eNB entity 120 may be connected through wired lines to one or more SGWs via the S1-U interface (“U” stays for “user plane”) and to the Mobility Management Entity 140 (MME) via the S1-MMME interface. The SGSN 150 and MME 140 are also referred to as serving core network (CN) nodes.
As shown above, the E-UTRAN consists of an eNodeB, providing the E-UTRA user plane (PDCP/RLC/MAC/PHY) and control plane (RRC) protocol terminations towards the user equipment (UE). The eNodeB (eNB) hosts the Physical (PHY), Medium Access Control (MAC), Radio Link Control (RLC) and Packet Data Control Protocol (PDCP) layers that include the functionality of user-plane header-compression and encryption. It also offers Radio Resource Control (RRC) functionality corresponding to the control plane. It performs many functions including radio resource management, admission control, scheduling, enforcement of negotiated uplink Quality of Service (QoS), cell information broadcast, ciphering/deciphering of user and control plane data, and compression/decompression of downlink/uplink user plane packet headers. The eNBs are interconnected with each other by means of the X2 interface.
FIG. 2 illustrates structure of a component carrier in LTE Release 8 and later releases. The downlink component carrier of the 3GPP LTE Release 8 is sub-divided in the time-frequency domain in so-called sub-frames each of which is divided into two downlink slots, one of which is shown in FIG. 2 as corresponding to a time period Tslot. The first downlink slot comprises a control channel region within the first OFDM symbol(s). Each sub-frame consists of a given number of OFDM symbols in the time domain, each OFDM symbol spanning over the entire bandwidth of the component carrier.
In particular, the smallest unit of resources that can be assigned by a scheduler is a resource block also called physical resource block (PRB). A PRB is defined as NsymbDL consecutive OFDM symbols in the time domain and NscRB consecutive sub-carriers in the frequency domain. In practice, the downlink resources are assigned in resource block pairs. A resource block pair consists of two resource blocks. It spans NscRB consecutive sub-carriers in the frequency domain and the entire 2·NsymbDL modulation symbols of the sub-frame in the time domain. NsymbDL may be either 6 or 7 resulting in either 12 or 14 OFDM symbols in total. Consequently, a physical resource block consists of NsymbDL×NscRB resource elements corresponding to one slot in the time domain and 180 kHz in the frequency domain (further details on the downlink resource grid can be found, for example, in Section 6.2 of the 3GPP TS 36.211, “Evolved universal terrestrial radio access (E-UTRA); physical channels and modulations (Release 12)”, version 12.1.0, March 2014, freely available at www.3gpp.org, which is incorporated herein by reference and denoted as “TS 36.211” in the following). While it can happen that some resource elements within a resource block or resource block pair are not used even though it has been scheduled, for simplicity of the used terminology still the whole resource block or resource block pair is assigned. Examples for resource elements that are actually not assigned by a scheduler include reference signals, broadcast signals, synchronization signals, and resource elements used for various control signal or channel transmissions.
The number of physical resource blocks NRBDL in downlink depends on the downlink transmission bandwidth configured in the cell and is at present defined in LTE as being from the interval of 6 to 110 (P)RBs. It is common practice in LTE to denote the bandwidth either in units of Hz (e.g. 10 MHz) or in units of resource blocks, e.g. for the downlink case the cell bandwidth can equivalently expressed as e.g. 10 MHz or NRBDL=50 RB.
Generally, it may be assumed that a resource block designates the smallest resource unit on an air interface of a mobile communication that can be assigned by a scheduler for transmitting data. The dimensions of a resource block may be any combination of time (e.g. time slot, sub-frame, frame, etc. for time division multiplex (TDM)), frequency (e.g. subband, carrier frequency, etc. for frequency division multiplex (FDM)), code (e.g. spreading code for code division multiplex (CDM)), antenna (e.g. Multiple Input Multiple Output (MIMO)), etc. depending on the access scheme used in the mobile communication system.
In 3GPP LTE Release 8 the downlink control signalling is basically carried by the following three physical channels:                Physical control format indicator channel (PCFICH) for indicating the number of OFDM symbols used for control signalling in a sub-frame (i.e. the size of the control channel region);        Physical hybrid ARQ indicator channel (PHICH) for carrying the downlink ACK/NACK associated with uplink data transmission; and        Physical downlink control channel (PDCCH) for carrying downlink scheduling assignments and uplink scheduling assignments.        
The PCFICH is sent from a known position within the control signalling region of a downlink sub-frame using a known pre-defined modulation and coding scheme. The user equipment decodes the PCFICH in order to obtain information about a size of the control signalling region in a sub-frame, for instance, the number of OFDM symbols. If the user equipment (UE) is unable to decode the PCFICH or if it obtains an erroneous PCFICH value, it will not be able to correctly decode the L1/L2 control signalling (PDCCH) comprised in the control signalling region, which may result in losing all resource assignments contained therein.
The PDCCH carries downlink control information, such as, for instance, scheduling grants for allocating resources for downlink or uplink data transmission. The PDCCH for the user equipment is transmitted on the first of either one, two or three OFDM symbols according to PCFICH within a sub-frame.
Physical downlink shared channel (PDSCH) is used to transport user data. PDSCH is mapped to the remaining OFDM symbols within one sub-frame after PDCCH. The PDSCH resources allocated for one UE are in the units of resource block for each sub-frame.
Physical uplink shared channel (PUSCH) carries user data. Physical Uplink Control Channel (PUCCH) carries signalling in the uplink direction such as scheduling requests, HARQ positive and negative acknowledgements in response to data packets on PDSCH, and channel state information (CSI).
User data (IP packets) to be transmitted over the communication network may be generated by the user application. They may include speech, video, text, or any other media possibly compressed and encapsulated into other protocols before forming the IP packets. The IP packets are in EUTRAN further processed on the PDCP layer resulting in addition of a PDCP header. The PDCP packets formed in this manner are further segmented and/or reassembled (reassembling being shown in the figure) into RLC packets to which an RLC header is added. One or more RLC packets are then encapsulated into a MAC packet including also a MAC header and padding, if necessary. The MAC packet is also called “transport block”. Thus, a transport block is from the point of view of the physical layer a packet of user data entering the physical layer. There are predefined transport block sizes (TBS) which may be used in LTE. The transport block is then within one transmission time interval (TTI) mapped onto the subframes on the physical layer (PHY). Details of the mapping of data starting with transport blocks up to the interleaving is shown in FIGS. 5.2.2-1 and 5.3.2-1 and described in the related description of the 3GPP TS 36.212, v.12.0.0, “Evolved universal terrestrial radio access (E-UTRA); Multiplexing and channel coding”, 2013, denoted in the following as TS 36.212 available freely at www.3qpp.org and incorporated herein by reference, for the uplink and downlink transmission of user data respectively. Furthermore, the physical channel mapping is described in detail in FIGS. 6.3-1 and FIGS. 5.3-1 for downlink and uplink, respectively, and the related description in the 3GPP TS 36.211.
The principle of link adaptation is fundamental to the design of a radio interface which is efficient for packet-switched data traffic. Unlike the early versions of UMTS (Universal Mobile Telecommunication System), which used fast closed-loop power control to support circuit-switched services with a roughly constant data rate, link adaptation in LTE adjusts the transmitted data rate (modulation scheme and channel coding rate) dynamically to match the prevailing radio channel capacity for each user.
For the downlink data transmissions in LTE, the eNodeB typically selects the modulation scheme and code rate (MCS) depending on a prediction of the downlink channel conditions. An important input to this selection process is the Channel State Information (CSI) feedback (mentioned above) transmitted by the User Equipment (UE) in the uplink to the eNodeB.
Channel state information is used in a multi-user communication system, such as for example 3GPP LTE to determine the quality of channel resource(s) for one or more users. In general, in response to the CSI feedback the eNodeB can select between QPSK, 16-QAM and 64-QAM schemes and a wide range of code rates. This CSI information may be used to aid in a multi-user scheduling algorithm to assign channel resources to different users, or to adapt link parameters such as modulation scheme, coding rate or transmit power, so as to exploit the assigned channel resources to its fullest potential. In order to select appropriate transmission parameter for a PDSCH transmission, the serving eNB relies on channel state information (CSI) reporting from the UE, which in LTE consists of the following:                Rank Indicator (RI)        Precoding Matrix Indicator (PMI)        Channel Quality Indicator (CQI)        
The CQI is used as input for the link adaptation algorithm in terms of MCS selection. The exact format of the CSI message depends on the reporting mode. The CQI may include separately coded wideband CQI and one or more subband CQIs, which are differentially coded with respect to the wideband CQI. The reporting mode is configurable by means of RRC signaling as described in 3GPP TS 36.331, v.12.1.0, 2014, “Radio Resource Control: Protocol specification”, freely available under www.3gpp.org. Reporting modes currently supported by LTE are defined in 3GPP TS 36.213,v.12.1.0, 2014, “Physical Layer Procedures” freely available under www.3.gpp.orq, and in particular in Section 6.2, e.g. information element “CQIreportConfig”.
The uplink and downlink resource grants (grants enabling the UE to transmit data in downlink and uplink, respectively) are transmitted from the eNodeB to the UE in a downlink control information (DCI) via PDCCH. The downlink control information may be transmitted in different formats, depending on the signaling information necessary. In general, the DCI may include: a resource block assignment (RBA) and modulation and coding scheme (MCS).
It may include further information, such as HARQ related information like redundancy version (RV), HARQ process number, or new data indicator (NDI); MIMO related information such as pre-coding; power control related information, etc.
As described above, in order to inform the scheduled users about their allocation status, transport format and other data-related information (e.g. HARQ information, transmit power control (TPC) commands), L1/L2 control signaling is transmitted on the downlink along with the data. L1/L2 control signaling is multiplexed with the downlink data in a subframe, assuming that the user allocation can basically change from subframe to subframe. It should be noted that user allocation might also be performed on a TTI (Transmission Time Interval) basis, where the TTI length can be in general a multiple of the subframes or correspond to a subframe. The TTI length may be fixed in a service area for all users, may be different for different users, or may even by dynamic for each user. Generally, the L1/2 control signaling needs only be transmitted once per TTI. Without loss of generality, the following assumes that a TTI is equivalent to one subframe.
In 3GPP LTE, assignments for uplink data transmissions, also referred to as uplink scheduling grants or uplink resource assignments, are also transmitted on the PDCCH. Generally, the information sent on the L1/L2 control signaling for assigning uplink or downlink radio resources (particularly LTE(-A) Release 10) can be categorized to the following items:                User identity, indicating the user that is allocated. This is typically included in the checksum by masking the CRC with the user identity. Then, the users (UEs) perform blind decoding by demasking the identities transmitted in the search space (i.e. in the resources configured as search space in which the respective terminals have to monitor the control information whether there is data for them).        Resource allocation information, indicating the resources (Resource Blocks, RBs) on which a user is allocated. Note, that the number of RBs on which a user is allocated can thus be dynamic. In particular, the number of the resource blocks (frequency domain) is carried by the resource allocation information. The position in the time domain (subframe) is given by the subframe in which the PDCCH is received and a predefined rule (the resources are allocated fixed number of subframes after the PDCCH subframe).        Carrier indicator, which is used if a control channel transmitted on a first carrier assigns resources that concern a second carrier, i.e. resources on a second carrier or resources related to a second carrier if carrier aggregation is applied.        Modulation and coding scheme that determines the employed modulation scheme and coding rate (length of the transport block to be coded).        HARQ information, such as a new data indicator (NDI) and/or a redundancy version (RV) that is particularly useful in retransmissions of data packets or parts thereof. In particular, new data indicator indicated whether the allocation is for an initial transmission of data or for a retransmission of data. Redundancy version indicates the coding applied to the retransmitted data (in LTE incremental redundancy combining is supported, meaning that each retransmission may include the data of the first transmission differently coded, i.e. may include parity bits which together with the already received transmission/retransmission(s) finally enable decoding).        Power control commands to adjust the transmit power of the assigned uplink data or control information transmission.        Reference signal information such as the applied cyclic shift and/or orthogonal cover code index, which are to be employed for transmission or reception of reference signals related to the assignment.        Uplink or downlink assignment index that is used to identify an order of assignments, which is particularly useful in TDD systems.        Hopping information, e.g. an indication whether and how to apply resource hopping in order to increase the frequency diversity.        CSI request, which is used to trigger the transmission of channel state information in an assigned resource.        Multi-cluster information, which is a flag used to indicate and control whether the transmission occurs in a single cluster (contiguous set of RBs) or in multiple clusters (at least two non-contiguous sets of contiguous RBs). Multi-cluster allocation has been introduced by 3GPP LTE-(A) Release 10.        
It is to be noted that the above listing is non-exhaustive, and not all mentioned information items need to be present in each PDCCH transmission depending on the DCI format that is used.
Downlink control information occurs in several formats that differ in overall size and also in the information contained in its fields. The different DCI formats that are currently defined for LTE are as follows and described in detail in 3GPP TS 36.212, v.12.0.0 “Multiplexing and channel coding”, section 5.3.3.1 (available at http://www.3gpp.org and incorporated herein by reference). For instance, DCI Format 0 is used for the transmission of resource grants for the PUSCH, using single-antenna port transmissions in uplink transmission mode 1 or 2.
In order for the UE to identify whether it has received a PDCCH transmission correctly, error detection is provided by means of a 16-bit CRC appended to each PDCCH (i.e. DCI). Furthermore, it is necessary that the UE can identify which PDCCH(s) are intended for it. This could in theory be achieved by adding an identifier to the PDCCH payload; however, it turns out to be more efficient to scramble the CRC with the “UE identity”, which saves the additional overhead. The CRC may be calculated and scrambled as defined in detail by 3GPP in TS 36.212, Section 5.3.3.2 “CRC attachment”, incorporated hereby by reference. The section describes how error detection is provided on DCI transmissions through a Cyclic Redundancy Check (CRC). A brief summary is given below. The entire payload is used to calculate the CRC parity bits. The parity bits are computed and attached. In the case where UE transmit antenna selection is not configured or applicable, after attachment, the CRC parity bits are scrambled with the corresponding RNTI.
Correspondingly, the UE descrambles the CRC by applying the “UE identity” and, if no CRC error is detected, the UE determines that PDCCH carries its control information intended for itself. The terminology of “masking” and “de-masking” is used as well, for the above-described process of scrambling a CRC with an identity. The “UE identity” mentioned above with which the CRC of the DCI may be scrambled can also be a SI-RNTI (System Information Radio Network Temporary Identifier), which is not a “UE identity” as such, but rather an identifier associated with the type of information that is indicated and transmitted, in this case the system information. The SI-RNTI is usually fixed in the specification and thus known a priori to all UEs.
The physical downlink control channel (PDCCH) carries e.g. scheduling grants for allocating resources for downlink or uplink data transmission. Multiple PDCCHs can be transmitted in a subframe. The PDCCH for the user equipments is transmitted on the first NsymbPDCCH OFDM symbols (usually either 1, 2 or 3 OFDM symbols as indicated by the PCFICH, in exceptional cases either 2, 3, or 4 OFDM symbols as indicated by the PCFICH) within a subframe, extending over the entire system bandwidth; the system bandwidth is typically equivalent to the span of a cell or component carrier. The region occupied by the first NsymbPDCCH OFDM symbols in the time domain and the NRBDL×NscRB subcarriers in the frequency domain is also referred to as PDCCH region or control channel region. The remaining NsymbPDCCH=2·NsymbDL−NsymbPDCCH OFDM symbols in the time domain on the NRBDL×NscRB subcarriers in the frequency domain is referred to as the PDSCH region or shared channel region (see below).
On a transport channel level, the information transmitted via the PDCCH is also referred to as L1/L2 control signaling (for details on L1/L2 control signaling see above).
The idea behind the concept of interference cancellation and interference suppression is that the effective signal to interference power ratio in the receiver can be increased if the interference fraction of the received signal can be removed or suppressed in the receiver. In order to achieve this, the knowledge of the type and strength of the interference is beneficial.
FIG. 3 illustrates the basic concept of interference cancellation in the receiver. A signal S is generated by the receiver and transmitted over a channel. During the transmission it is superimposed by interference I and noise N. This results in a disturbed signal which is inputted to a receiver and which may lead to some bit errors in the demodulator. In order to improve the reception and in particular, the bit error rate resulting from demodulation and decoding, interference cancellation (marked by a dashed rectangle in FIG. 3) may be applied. In particular, an interference estimation I′ available in the receiver is used to recover signal S′ which is further used as an input for the demodulator reducing therewith the bit error rate. In this example, the recovery is achieved by subtracting the estimated interference signal I′ form the received signal S+N+I. The performance of interference cancellation strongly depends on the accuracy of the interference estimation I′. In case of a very inaccurate interference estimation which corresponds to a large difference between I and I′, it could even result in an increased disturbance of the demodulator input yielding an increased bit error rate.
The interference I is determined by a combination of some transmission (interference) parameters. The accuracy of the interference estimation I′ increases with the amount of information regarding the interference parameters that is available on the receiver side.
In order to translate the SINR increased by successful interference cancellation or suppression into actual throughput gains, the expected SINR gain has to be taken into account for the modulation and coding scheme (MSC) selection prior to the data transmission on the eNB side. The SINR prediction is usually conducted by a combination of channel quality reporting from the UE and the evaluation of ACK/NACK statistics in the eNB. This approach is also known as OLLA (outer-loop link adaptation) within the scope of 3GPP TR 36.866 v12.0.0, March 2014, “Study on Network-Assisted Interference Cancellation and Suppression (NAIC) for LTE” (referred to as “NAICS technical report” in the following). The OLLA represents a control loop that uses the channel quality reports from the UE and the observation of transmission errors as input for the MCS selection. The observed transmission error rate (ACK/NACK statistics) can be seen here as a metric for the accuracy of the proper MCS selection. If the number of transmission errors is too high, the MCS selection can be considered as too aggressive; and if the number of transmission errors to very low, the MCS selection can be considered as too conservative. A schematic representation of the link adaptation is shown in FIG. 4. In particular, the link adaptation unit applies a link adaptation algorithm by evaluating the statistics of the (positive or negative) acknowledgements and channel quality information (CQI) received from the terminal.
FIG. 5 shows a typical scenario with interference from a single dominant interfering cell. UEs 501, 502, 503 and 504 are served by an eNB A and experience interference from eNB B. UE 501 and UE 502 experience weak interference from eNB B since they are far away from the interference source (eNB B), while UE 503 and UE 504 experience strong interference from eNB B. The dashed circle 500 indicates the area, in which the interference from the eNB B is dominant for the terminals served by the eNB A. For the purpose of improving the reception quality in the terminals located within the area 500, the interference cancellation and thus also the accuracy of the interference estimation may be essential.
Recently, 3GPP initiated a study item concerning network assisted interference cancellation and suppression (NAICS) for the downlink in 3GPP LTE systems. Details are described in the NAICS technical report cited above. Based thereon, a subsequent work item is supposed to specify inclusion of the network assisted interference cancellation into the standard, as can be seen from RP-140519, “New work item proposal for network assistance interference cancellation and suppression for LTE”, 3GPP RAN#63, March 2014, referred to as “NAICS work item” in the following.
The parameters in an LTE system which influence the interference (interference parameters) comprise                Position of reference signals (pilot) within the resource grid of the interfering transmission (e.g. by eNB B of FIG. 5),        Effective interference channel including precoding on the interference transmitter side,        Interferer resource allocation in terms of allocated resources (PRBs, CFI, etc.),        Number of spatial transmission layers of the interfering transmission,        Modulation order of the interfering transmission,        Channel coding parameters of the interfering transmission (code rate, redundancy version, etc.).        
The amount of required interference information depends hereby on the receiver type. The receiver types investigated at 3GPP range from receivers that suppress the interference by means of spatial filtering of the sum signal to receivers that perform the complete decoding of code words transmitted by the interference source.
A receiver that performs merely an interference suppression by means of spatial filtering of the received signal (e.g. E-LMMSE-IRC in the NAICS technical report, Section 7.2) requires only information about the effective interference channel (including precoding on the interference transmitter side) per spatial layer, while information about modulation and coding scheme, redundancy version, etc. are not required.
On the other hand, a receiver that performs interference cancellation either on symbol (SL-IC, cf. NAICS technical report, Section 7.4) or on codeword level (CW-IC, cf. NAICS technical report, Section 7.4) requires a significantly extended amount of interference information. In particular, on the receiver side it has to be known which modulation symbol was transmitted in order to perform effective interference cancellation as show in FIG. 4. A detailed description of the receiver types studied at 3GPP RAN1 within the scope of NAISC is given in the NAICS technical report cited above. Accordingly, also different approaches can be considered for obtaining interference parameters in LTE system:                Blind detection: The interference parameters are estimated within the receiver by means of hypothesis testing. This approach does not involve any network assistance but constitutes increased computation complexity in the receiver.        Overhearing of control signals (DCI, reference signals, etc.) from interfering eNB: The interference parameters are determined in the receiver by listening to existing control signals from the interfering cell itself.        L1 signalling from interfering eNB: This approach addresses the introduction of new L1 (physical layer) signalling for the provision of interference information. This control information is to be transmitted by the interfering eNB.        Higher-layer signalling from serving eNB: Interference information is provided to the interference victim UE by the serving eNB by means of downlink control messages on MAC layer or above.        L1 signalling from serving eNB: Interference information is provided to the interference victim UE by the serving eNB by means of downlink control signalling on the physical layer.        
The network assisted interference handling (cancellation or suppression) enables achieving higher reception quality in terms of SINR. The receiving devices such as UE report their reception quality to the network so that the network can allocate resources accordingly. The reception quality depends on whether interference cancellation was performed or not and under which interference hypothesis the interference cancellation was performed. In particular, as explained above, the interference parameters may indicate various transmission characteristics of the interfering nodes. Such characteristics form then an interference hypothesis, i.e. an estimation of the interference on the basis of such parameters. According to the estimated interference, the received signal is processed by applying the interference cancellation and suppression. The network receives then channel quality report. In order to reliably perform link adaptation, efficient signalling of the feedback concerning channel quality is necessary.